1. Field of the Invention
The present invention relates to transflective liquid-crystal display devices of the super twisted nematic (STN) type, and more particularly, the present invention relates to a transflective liquid-crystal display device having excellent display features not only in a reflection mode but also in a transmission mode.
2. Description of the Related Art
Almost all portable phones and portable information terminals are currently equipped with a liquid crystal display device, and most of these portable electronic apparatuses are currently equipped with a transflective liquid-crystal display device.
Known transflective liquid-crystal display devices, either of the active matrix type or the passive matrix type, are liquid crystal display devices having an external transflector and having a structure in which one of a pair of mutually opposing glass substrates (i.e., the substrate away from the observer) sandwiching a liquid crystal layer therebetween has a transflective sheet, a retardation film, and a polarizer on the lower surface thereof in this order, and the other glass substrate (i.e., the substrate close to the observer) has another retardation film and another polarizer on the upper surface thereof in this order.
Since developments to make color displays and requirements for higher density of display pixels, in particular, cause the above liquid crystal display devices to suffer from blurred display problems due to parallax, color mixing with unwanted colors, and the like, a liquid crystal display device having a built-in transflector, where the transflector is provided on the inner surface of one of the pair of glass substrates (i.e., the substrate away from the observer), has been mainly used.
FIG. 9 illustrates a partial sectional structure of the known liquid crystal display device having a built-in transflector. This transflective liquid-crystal display device has a pair of glass substrates 71 and 72 and a transflector 75, on the upper surface of the lower glass substrate 71 (away from the observer ob), formed by a layer 73 having a concave-convex upper surface so as to provide a diffuse reflection and a high-reflectivity film 74 which is made of Al-based or Ag-based metal film, or the like, and which is stacked on the layer 73. Furthermore, the transflector 75 has a color filter layer 76, a planarizing layer 77, transparent electrodes 78a, and an alignment film 79a formed on the upper surface thereof. The foregoing layer 73 having a concave-convex upper surface has fine concavities and convexities formed in a random manner by treating the upper surface of a transparent substrate such as a glass substrate by sand blasting, etching, or the like. Also, the high-reflectivity film 74 formed on the foregoing layer 73 has fine concavities and convexities 74c on its surface, whose sectional shape exhibits a continuous curve having continuous slopes.
While the lower glass substrate 71 has such a transflector, the upper glass substrate 72 (close to the observer ob) has transparent electrodes 78b and an alignment film 79b formed on the lower surface thereof so as to serve as a counter substrate. The substrate 71 with the transflector and the counter substrate 72 are bonded to each other with a sealant having a loop-like shape in plan view (not shown), and a liquid crystal layer 80 is formed by injecting liquid crystal inside and sealing it in the space enclosed by the pair of glass substrates 71 and 72 and the sealant, so as to provide a liquid crystal cell 81. The upper and lower alignment films 79b and 79a are aligned so that the alignment directions of liquid crystal molecules in the liquid crystal layer 80 are twisted by about 220 to 250 degrees.
Also, the liquid crystal cell 81 has an optical film 82b, made of at least one retardation film, and a polarizer 83b stacked on the upper surface thereof (close to the observer ob) in that order. In addition, the liquid crystal cell 81 has an optical film 82a, formed by a plurality of retardation films, and a polarizer 83a stacked on the lower surface thereof (away from the observer ob) in that order. Furthermore, a backlight unit 100 is disposed below the polarizer 83a. 
The backlight unit 100 is formed by a transparent light guide plate 101, a reflecting tube 103 which has a U-shaped cross section and which is disposed so as to oppose one of the side surfaces of the light guide plate 101, a white light source 102, such as a cold cathode fluorescent lamp (CCFL), a white light emitting diode, or the like, housed in the reflecting tube 103, and a reflecting plate 104 disposed on an external surface (the lower surface in the figure) of the light guide plate 101.
The transflective liquid-crystal display device having the above-mentioned backlight unit 100 is used as, for example, a display portion of a portable phone, and the display portion is used by switching between a reflection mode and a transmission mode, wherein, in the reflecting mode, the reflective liquid-crystal display device uses sunlight or external light as a light source and, in the transmission mode, the transmissive liquid-crystal display device uses the backlight unit 100 as a light source.
The transflector 75 exhibiting the foregoing diffuse reflection has the above-described fine concavities formed in a random manner so as to provide a reflected luminance characteristic, shown by a curve (1) in FIG. 10, which exhibits an approximately symmetric distribution with respect to the angle of its specular reflecting direction (an approximate Gaussian distribution), or by another curve (2) in FIG. 10 which exhibits a combined distribution in which the above distribution is added with its specular reflection component. In the transflector 75 having the diffuse reflection exhibiting the foregoing approximately symmetric distribution (approximate Gaussian distribution), the metal thin film high-reflectivity film 74 formed on the foregoing layer 73 has the fine concavities and convexities 74c formed in a random manner on the surface thereof, as shown in FIG. 14, whose sectional shape exhibits a continuous curve having continuous slopes, that is, joining portions (boundaries) 74d between adjacent concavities are formed so as to be convex curves. In the transflector 75 having the diffuse reflection exhibiting the foregoing combined distribution, the foregoing layer 73 has a flat portion formed at a part of the fine concavities formed on the upper surface thereof so that the metal thin film on the flat portion has a reflection characteristic. FIG. 11 illustrates a method for measuring the reflected luminance characteristic of the transflector shown in FIG. 10. With this method, when the upper surface of the transflector 75 is irradiated with incident light (external light) L1 at an incident angle xcex81 (an angle from the normal H), a photo detector 105 detects reflected light R1, which is part of the incident light L1 reflected at the foregoing surface, at an acceptance angle xcex8a from the normal H (0xc2x0). A curve (7) in FIG. 13 shows the measured relationship between the reflected luminance vs. an acceptance angle, measured by varying the acceptance angle xcex8a from the normal H (0xc2x0) to, for example, 60xc2x0 while the angle xcex81, which indicates the specular reflecting direction with respect to the surface of the transflector, is set as a center angle.
An example reflector having transflectivity and used in the transflective liquid-crystal display devices is a) the transflector 75 using the metal thin film (high-reflectivity film) 74 with a film thickness of 5 to 40 nm, as shown in FIG. 9, so as to provide an appropriate transmittivity in the visible light region; b) a transflector having a plurality of apertures in a metal film; or the like.
FIG. 12A illustrates a partial sectional structure of another example liquid-crystal display device having the foregoing transflector b), that is, a transflector 75a formed by a layer 73a having concavities and convexities and formed on the surface thereof and by a metal film 74a having a plurality of apertures 74b and formed on the layer 73a. FIG. 12B is a plan view illustrating the positional relationships between the apertures 74b and the upper and lower transparent electrodes 78a and 78b of the liquid crystal display device when viewed from the observer""s side. Like parts are identified by the same reference numerals in FIG. 9, and their description is omitted. Also, although not shown in the figure, the liquid crystal display device in FIG. 12 has a backlight unit, similar to that in FIG. 9, below a liquid crystal cell 81a. 
In this liquid crystal display device, in order to optimize the brightness and contrast in a transmission mode while maintaining the necessary reflected luminance in a reflection mode, the areas and shapes of the apertures 74b are set so that the transmittance of the overall liquid crystal panel formed by a polarizer, an optical film, and a liquid crystal cell is normally about 1% to 4%.
However, improved characteristics are required for known transflective liquid-crystal display devices including various types of reflector.
For example, since the device including the transflector 75 using the high-reflectivity film 74 as mentioned in the foregoing a) utilizes the transflectivity of the high-reflectivity film 74, in a transmission mode, illuminating light passing through the high-reflectivity film 74 causes its color to change, and, in a reflection mode, the spectral reflectivity of the high-reflectivity film 74 causes the reflected luminance to deteriorate as a whole or the color to change. In addition, since the transmittance of the high-reflectivity film 74 is required to be strictly controlled (for example, the transmittance of the metal thin film itself is required to be 15% (xc2x15% or less) to 25% (xc2x15% or less) in the visible light region so as to maintain the display characteristics required for a display component), it is difficult to fabricate the high-reflectivity films 74, controlled as described above, in a well reproducible manner on a mass-production basis.
In addition, when used as a portable information terminal such as a portable phone, the known liquid crystal display device serving as a display component is often observed from a particular direction and is also required to an ambient-light collecting ability in the viewing direction. However, since the conventional transflectors of any type described above have reflected luminance characteristic whose curve is approximately symmetrical with those observed from a direction from which the observer does not view (a direction opposite to the viewing direction of the observer), it is difficult to improve the reflected luminance at the observer""s side with respect to the normal of the liquid crystal display device, thereby resulting in dark display in the viewing side of the observer.
Furthermore, in known transflective liquid-crystal display devices, when the retardation (xcex94nd: where xcex94n and d are anisotropy of refractive index and the lay thickness of liquid crystal, respectively) of the liquid crystal cell is set to be equal to 740 nm or greater, for example, at a measuring wavelength of 589 nm, the characteristic is excellent in a transmission mode; however, in a reflective mode, the effective optical depth of the liquid crystal cell increases since incident light passes through the liquid crystal cell twice, thereby causing its display to be dark. Also, in this case, since the anisotropy of refractive index of the liquid crystal in use becomes large, its chromatic dispersion (wavelength dependency) inevitably becomes large, thereby leading to problems in that its color tends to change when the viewing angle changes and thus its color reproducibility deteriorates.
Moreover, although the optical films and polarizers are disposed above and below the liquid crystal cell so as to have optical axes (in general, absorption axes for the polarizers and slow axes for the retardation films) at respective predetermined angles, it is difficult to obtain a bright display having good color reproducibility in both the reflective and transmissive modes.
Although various transflective liquid-crystal display devices have been proposed in order to solve the above described problems, a transflective liquid-crystal display device which, in a reflective mode, offers a bright display especially over the viewing angle range of an observer, and good color reproducibility, and which, in a transmissive mode, also offers a bright display and good color reproducibility has not been achieved.
The present invention has been made to solve the foregoing problems. Accordingly, it is an object of the present invention to provide an STN-type transflective liquid-crystal display device which, in a reflective mode, offers a bright display, especially in the viewing angle range of an observer, and good color reproducibility, and which, in a transmissive mode, offers display being also bright and having good color reproducibility.
Also, it is another object of the present invention to provide an STN-type transflective liquid-crystal display device, performing a duty of about {fraction (1/200)} (corresponding to matrix driving with 200 scanning lines), which, in a reflective mode, offers a bright display, especially in the viewing angle range of an observer, and having good color reproducibility, and which, in a transmissive mode, also offers a bright display and color reproducibility.
To achieve to the above objects, the transflective liquid-crystal display device according to the present invention has a structure which will be described below.
A transflective liquid-crystal display device according to the present invention comprises a liquid crystal cell, the liquid crystal cell comprising: a liquid crystal layer; a pair of mutually opposing transparent substrates sandwiching the liquid crystal layer; transparent electrodes and an alignment film formed close to the inner surface of one of the transparent substrates in that order; other transparent electrodes and another alignment film formed close to the inner surface of the other transparent substrate in that order; and a transflector disposed close to the one transparent substrate. The transflective liquid-crystal display device further comprises a first optical compensating plate and a first polarizer formed close to the outer surface of the other transparent substrate in that order; a second optical compensating plate and a second polarizer formed close to the outer surface of the one transparent substrate in that order; and an illuminator which is disposed close to the outer surface of the second polarizer formed close to the outer surface of the one transparent substrate and which emits illuminating light toward the liquid crystal cell.
The liquid crystal layer comprises a liquid crystal composition which has a positive dielectric anisotropy, which is twisted by about 220 to 260 degrees, and which is sandwiched by the pair of transparent substrates.
Also, the transflector comprises a high-reflectivity film having a plurality of fine apertures therein and the high-reflectivity film comprises a diffuse reflection surface, on the surface thereof, whose reflected luminance characteristic is controlled.
The transflector comprising the high-reflectivity film having the reflected luminance characteristic controlled as described above is disposed close to the inner surface of the one transparent substrate, the transflector or a film-like transflector comprising the high-reflectivity film formed on the upper surface of a base member or a resin film having concavities and convexities on the upper surface thereof may be laminated close to the inner surface of the one transparent substrate.
When the transflector comprising the high-reflectivity film having the reflected luminance characteristic controlled as described above is disposed close to the outer surface of the one transparent substrate, the transflector or the film-like transflector comprising the high-reflectivity film formed on the upper surface of the base member or the resin film having concavities and convexities on the upper surface thereof may be laminated close to the outer surface of the one transparent substrate.
The diffuse reflection surface of the high-reflectivity film is controlled, for example, for its reflected luminance characteristic not to exhibit a typical approximate Gaussian distribution (that is, its reflected luminance characteristic does not necessarily exhibit an approximate symmetric distribution with respect to the acceptance angle of its specular reflecting direction) or so as to have a distribution deviated from the typical approximate Gaussian distribution. More particularly, the diffuse reflection surface of the high-reflectivity film is controlled such that its reflected luminance characteristic exhibits a distribution having a substantially flat portion in its high reflected luminance region, preferably over a majority or substantially the entirety of the high reflected luminance region. One definition of the high reflected luminance region is the region of reflectance in which the reflectance of the impinging light is about 90% or greater of the maximum reflectance. An alternate definition is the region in which the contrast of the device is about 90% or greater of the maximum contrast.
According to the present invention, in the liquid crystal display device having a structure in which the foregoing liquid crystal layer includes the liquid crystal composition which has a positive dielectric anisotropy and which is sandwiched by the foregoing pair of transparent substrates while being twisted by 220 to 260 degrees, and the foregoing liquid crystal cell has respective optical compensating plates and polarizers thereon and thereunder, the foregoing transflector comprising the high-reflectivity film having a plurality of fine apertures is disposed close to the one transparent substrate of the liquid crystal cell and the high-reflectivity film is provided with the diffuse reflection surface, on the surface thereof, having the reflected luminance characteristic controlled so as not to exhibit an approximate symmetric distribution or so as to have a distribution deviated from the typical approximate Gaussian distribution, whereby, in a reflection mode, a reflection display having a high reflected luminance over a wide viewing angle is achieved and, in a transmissive mode, an excellent transmission display in which the transmittivity of transmitted light does not vary over a wide viewing angle is obtained.
Such advantages can be obtained in an STN-type transflective liquid-crystal display device performing a duty of about {fraction (1/200)} (corresponding to matrix driving with 200 scanning lines).
In the transflective liquid-crystal display device according to the present invention, the liquid crystal cell may comprise a color filter layer close to the inner surface of either one of the pair of transparent substrates. With such a transflective liquid-crystal display device, in the reflection mode, a bright color display having especially good contrast in the observing angle range and good color reproducibility is obtained, and also, in the transmission mode, a bright color display having good contrast and good color reproducibility is obtained.
In the transflective liquid-crystal display device according to the present invention, the color filter layer is preferably formed on the high-reflectivity film of the transflector.
In the transflective liquid-crystal display device according to the present invention, the first optical compensating plate formed close to the outer surface of the other transparent substrate comprises first and second retardation films, and the second optical compensating plate formed close to the outer surface of the one transparent substrate comprises a third retardation film,
the liquid crystal composition which is twisted by about 220xc2x0 to 260xc2x0 and whose transmitted luminance vs. voltage characteristic has a steepness index lying in the range from about 1.030 to 1.075 is used in the liquid crystal layer, and the liquid crystal cell has a birefringent retardation (xcex94ndLC) lying in the range from about 690 to 735 nm (at a temperature of 25xc2x0 C. and the measuring wavelength of 589 nm),
when an alignment direction a of the alignment film close to the other transparent substrate and an alignment direction b of the alignment film close to the one transparent substrate are viewed from above, a reference direction X lies between the alignment directions a and b, passes through the intersection O of the alignment directions a and b, and also extends along a line bisecting the inner angle formed by the alignment directions a and b,
the first retardation film has a birefringent retardation (xcex94ndRF1) lying in the range from about 150 to 190 nm at a measuring wavelength of 546 nm, and also has a slow axis xcex2 which forms an angle (xcfx86RF1) lying in the range from about 65 to 95 degrees with respect to the reference direction X in the counterclockwise direction when viewed from above,
the second retardation film has a birefringent retardation (xcex94ndRF2) lying in the range from about 350 to 400 nm at a measuring wavelength of 546 nm, and also has a slow axis xcex3 which forms an angle (xcfx86RF2) lying in the range from about 90 to 135 degrees with respect to the reference direction X in the counterclockwise direction when viewed from above,
the first polarizer has an absorption axis a which forms an angle (xcfx86pol1) lying in the range from about 35 to 55 degrees with respect to the reference direction X in the counterclockwise direction when viewed from above,
the third retardation film has a birefringent retardation (xcex94ndRF3) lying in the range from about 115 to 135 nm at a measuring wavelength of 546 nm, and also has a slow axis xcex4 which forms an angle (xcfx86RF3) lying in the range from about 55 to 85 degrees with respect to the reference direction X in the counterclockwise direction when viewed from above,
the second polarizer has an absorption axis xcex5 which forms an angle (xcfx86pol2) lying in the range from about 10 to 40 degrees with respect to the reference direction X in the counterclockwise direction when viewed from above, and
an angle formed by the slow axis xcex4 of the third retardation film and the absorption axis xcex5 of the second polarizer is set in the range from about 30 to 50 degrees.
In the transflective liquid-crystal display device, by setting the optical conditions of the liquid crystal layer, the liquid crystal cell, the first to third retardation films, and the first and second polarizers in the ranges according to the present invention, a bright display having especially good contrast in the observing angle range and good color reproducibility is obtained in the reflection mode, and also a bright display having good contrast and good color reproducibility is obtained in the transmission mode.
In the transflective liquid-crystal display device according to the present invention, the steepness index xcex (=V90/V10) of the liquid crystal preferably lies in the range from about 1.030 to 1.060 when driven by a typical passive-matrix voltage averaging method (so-called APT drive method).
In the transflective liquid-crystal display device according to the present invention, the steepness index preferably lies in the range from about 1.040 to 1.075 when driven by another drive method such as a multi-line addressing method (MLA drive method).
In the transflective liquid-crystal display device according to the present invention, the liquid crystal composition used in the liquid crystal layer is preferably twisted by about 240 to 250 degrees.
When set in these ranges, preferable results are obtained.
In the transflective liquid-crystal display device according to the present invention, the birefringent retardation (xcex94ndLC) of the liquid crystal cell preferably lies in the range from about 700 to 730 nm (at a temperature of 25xc2x0 C. and the measuring wavelength of 589 nm), and more preferably in the range from about 710 to 725 nm.
In the transflective liquid-crystal display device according to the present invention, the angle (xcfx86pol1) formed by the absorption axis xcex1 of the first polarizer with respect to the reference direction X preferably lies in the range from about 40 to 50 degrees in the counterclockwise direction when viewed from above (the observer""s side).
In the transflective liquid-crystal display device according to the present invention, the birefringent retardation (xcex94ndRF1) of the first retardation film preferably lies in the range from about 155 to 185 nm at a measuring wavelength of 546 nm, and more preferably in the range from about 165 to 175 nm, and also the angle (xcfx86RF1) formed by the slow axis xcex2 of the first retardation film with respect to the reference direction X lies in the range from about 70 to 90 degrees in the counterclockwise direction when viewed from above (the observer""s side), and more preferably in the range from about 76 to 80 degrees.
In the transflective liquid-crystal display device according to the present invention, the birefringent retardation (xcex94ndRF2) of the second retardation film preferably lies in the range from about 360 to 400 nm at a measuring wavelength of 546 nm, and more preferably in the range from about 370 to 380 nm, and also the angle (xcfx86RF2) formed by the slow axis xcex3 of the second retardation film with respect to the reference direction X preferably lies in the range from about 100 to 130 degrees in the counterclockwise direction when viewed from above (the observer""s side), and more preferably in the range from about 110 to 120 degrees.
In the transflective liquid-crystal display device according to the present invention, the birefringent retardation (xcex94ndRF3) of the third retardation film preferably lies in the range from about 120 to 130 nm at a measuring wavelength of 546 nm, and more preferably at about 125 nm.
In the transflective liquid-crystal display device according to the present invention, the angle (xcfx86pol2) formed by the absorption axis xcex5 of the second polarizer with respect to the reference direction X preferably lies in the range from about 20 to 30 degrees in the counterclockwise direction when viewed from above (the observer""s side).
In the transflective liquid-crystal display device according to the present invention, when the alignment direction a of the alignment film close to the other transparent substrate and the alignment direction b of the alignment film close to the one transparent substrate are viewed from above (the observer""s side), the reference direction X is related to an angle formed by the foregoing alignment directions a and b. For example, when the viewing direction of the liquid crystal cell is toward the proximal side (in the direction of six o""clock on the clock dial) and also the liquid crystal composition twisted in a left spiral manner is chosen, the reference direction X corresponds close to the direction of about three o""clock on the clock dial which lies close to the rubbing-alignment process direction of the one transparent substrate (the lower transparent substrate), and when the viewing direction of the liquid crystal cell is toward the far side (in the direction of twelve o""clock on the clock dial), the reference direction X corresponds close to the direction of about nine o""clock on the clock dial which lies close to the rubbing-alignment process direction of the other transparent substrate (the upper transparent substrate).
In the transflective liquid-crystal display device according to the present invention, the optical conditions of the first to third retardation films and the first and second polarizers, that is, the relationships among the absorption axes of the first and second polarizers, the slow axes of the first to third retardation films, and so forth, can be modified as needed. In particular, since the arrangements of the absorption axes of the polarizers most dominantly affect the display characteristics of the liquid crystal display device, for example, when the absorption axis of the first polarizer close to the observer""s side is turned clockwise (or counterclockwise), the slow axis of the third retardation film and the absorption axis of the second polarizer, both being laminated on the under surface of the liquid crystal cell, are also modified so as to be turned clockwise (or counterclockwise) in correspondence to the above arrangements. In this case, by modifying the above arrangements, while substantially maintaining the angular relationship between an upper optical film, including the first polarizer, the first and second retardation films, and so forth, and a lower optical film including the second polarizer, the third retardation film, and so forth, good results can be obtained.
The first or second polarizer used in the present invention is selected, as needed, from high-contrast polarizers respectively having a non-glare process and an antireflection process applied on the corresponding surfaces thereof. The first and second retardation films used in the present invention are selected, as needed, from films which are obtained by drawing polymer films such as polycarbonate, polyarylate, and the like by controlling the drawing in a single axis. Also, a so-called Z-type retardation film whose refractive index is controlled in its thickness direction can be used (its Z-coefficient lying in the range from about about 0.2 to 0.6). These retardation films have an advantage in improving viewing angle characteristics.
By disposing the transflector comprising the high-reflectivity film including the diffuse reflection surface on the upper surface thereof, whose reflected luminance characteristic is controlled, in the liquid crystal cell, the transflective liquid-crystal display device according to the present invention offers a reflection display having a high reflected luminance at an observing angle varying in the range from about 5 to 45 degrees (about 5 to 45 degrees from the normal) in the reflection mode, and also offers an excellent transmission display in which the transmittivity of transmitted light does not vary over a predetermined angle range in the transmission mode.
The diffuse reflection surface of the transflector having the foregoing characteristics does not have a reflected luminance vs. acceptance angle characteristic which exhibits a conventional approximately symmetric curve (approximate Gaussian distribution, curve (7) shown in FIG. 13) with respect to the acceptance angle of the specular reflecting direction, but instead exhibits a curve (curve (3) shown in FIG. 13) having a substantially flat portion in its high reflected luminance with respect to an acceptance angle.
The transflector including the diffuse reflection surface having the foregoing characteristic is achieved, for example, by forming the high-reflectivity film on the base member having a plurality of fine concavities or convexities on the upper surface thereof, forming a plurality of fine concavities or convexities on the upper surface of the high-reflectivity film, i.e., on the diffuse reflection surface, and, as shown in FIG. 16, forming the sectional shape of the diffuse reflection surface 36c so as to have curved surfaces whose slopes are discontinuous and also forming a plurality of fine concave surfaces or fine convex surfaces without substantially leaving spaces. Also, the diffuse reflection surface having the foregoing characteristic is achieved such that each fine concave surface 36a or convex surface is formed so as to have an asymmetrical sectional shape and joining portions (boundaries) 36d between the adjacent concave surfaces 36a are processed by a method such as lithography, beam processing, or mechanical pressing so as not to have dull peaks.
FIG. 16 illustrates an example of the diffuse reflection surface 36c in which the plurality of fine concave surfaces (fine concavities) 36a are formed without substantially leaving spaces. The fine apertures formed in the high-reflectivity film are not shown in FIG. 16.
It is known that an acceptance angle range in which a high level of reflected luminance is achieved is about twice the tilt angle of the fine concave surface (or fine convex surface).
On the other hand, in a known transflector having a reflected luminance vs. acceptance angle characteristic exhibiting an approximate Gaussian distribution (a curve (7) shown in FIG. 13), concavities and convexities 74c are formed on the upper surface of a metal film 74a, as shown in FIG. 15, and have a sectional shape exhibiting a continuous curve having continuous slopes, that is, joining portions (boundaries) 74d between the adjacent concavities have curved surfaces. Apertures formed in the metal films are not shown in FIG. 15.
FIG. 13 shows the relationship between the reflected luminance and an acceptance angle obtained such that, when the upper surface of the transflector 75 is irradiated with incident light (external light) L1 at an incident angle xcex81 (an angle from the normal H), a photo detector 105 detects reflected light R1, which is part of the incident light L1 reflected at the foregoing surface, at an acceptance angle xcex8a from the normal H (0xc2x0), and the reflected luminance is measured by varying the acceptance angle xcex8a from the normal H (0xc2x0) to, for example, 60xc2x0 while the angle xcex81, which indicates the specular reflecting direction with respect to the surface of the transflector, is set as a center angle.
The base member used in the transflector according to the present invention may be controlled so as to have a surface structure having an asymmetrical sectional shape with respect to the normal of the substrate. With this arrangement, the high-reflectivity film formed on the base member can be controlled so as to have a high reflected luminance only over a desired observing angle range. The surface structures of the base member and the high-reflectivity film are preferably formed so as to have the concavities or convexities without substantially leaving spaces.
The transflector having the above described structure can be formed, selectively as needed, by photolithography and using a metal film provided with electrolytic plating, by using an energy beam whose power is controllable, by using a mechanical method for forming desired shapes, or the like.
The foregoing high-reflectivity film is preferably made from an Al-based or Ag-based metal film. Alternatively, it may be made from, for example, an Al-Nd-based metal film.
The aperture ratio of each fine aperture in the foregoing high-reflectivity film preferably lies in the range from about 15% to 35% with respect to the area of one pixel pitch of the liquid crystal cell.
The high-reflectivity film of the transflective liquid-crystal display device preferably has a reflected luminance characteristic in which the diffuse reflection surface has a high reflected luminance region with a substantially flat portion.
The diffuse reflection surface may have curved surfaces with slopes that are discontinuous between adjacent curved surfaces and that have substantially no space between the adjacent curved surfaces. Preferably, each curved surface also has an asymmetrical sectional shape.
In another embodiment of the invention, a method of fabricating a transflective liquid-crystal display device comprises introducing a liquid crystal layer between a first and a second transparent substrate and limiting the liquid crystal layer to having a liquid crystal composition of a positive dielectric anisotropy and which is twisted by about 220 to 260 degrees. The method also comprises forming a transflector on an inner surface of the first transparent substrate, forming a plurality of fine apertures in a high-reflectivity film of the transflector thereby forming a diffuse reflection surface on the transflector, controlling a reflected luminance characteristic of the diffuse reflection surface such that the diffuse reflection surface has a high reflected luminance region with a substantially flat portion, and planarizing the transflector by covering the fine apertures with a planarizing film. Additionally, the method also comprises forming a first set of transparent electrodes and a first alignment film on an inner surface of the transflector, a first optical compensating plate and a first polarizer on an outer surface of the first transparent substrate, a second set of transparent electrodes and a second alignment film on an inner surface of the second transparent substrate, and a second optical compensating plate and a second polarizer on an outer surface of the second transparent substrate and placing a backlight proximate to an outer surface of the second polarizer.
The method may also comprise forming a color filter layer proximate to the inner surface of either of first and second transparent substrates and more preferably comprise forming the color filter layer on the high-reflectivity film of the transflector prior to planarizing the transflector.
The method preferably comprises incorporating first and second retardation films in the first optical compensating plate and incorporating a third retardation film in the second optical compensating plate; limiting a steepness index of a transmitted luminance vs. voltage characteristic of the liquid crystal composition of the liquid crystal layer to about 1.030 to 1.075 and limiting a birefringent retardation (xcex94ndLC) from about 690 to 735 nm at a measuring wavelength of 589 nm; limiting a first alignment direction a of the first alignment film and a second alignment direction b of the second alignment film such that as viewed from above, a reference direction X lies between the alignment directions a and b, passes through an intersection O of the alignment directions a and b, and also extends along a line bisecting an inner angle formed by the alignment directions a and b; limiting a birefringent retardation (xcex94ndRF1) of the first retardation film from about 150 to 190 nm at a measuring wavelength of 546 nm, and a slow axis xcex2 which forms an angle (xcfx86RF1) from about 65 to 95 degrees with respect to the reference direction X in a counterclockwise direction when viewed from above; limiting a birefringent retardation (xcex94ndRF2) of the second retardation film from about 350 to 400 nm at a measuring wavelength of 546 nm, and a slow axis xcex3 which forms an angle (xcfx86RF2) from about 90 to 135 degrees with respect to the reference direction X in the counterclockwise direction when viewed from above; limiting an absorption axis xcex1 of the first polarizer which forms an angle (xcfx86pol1) from about 35 to 55 degrees with respect to the reference direction X in the counterclockwise direction when viewed from above; limiting a birefringent retardation (xcex94ndRF3) of the third retardation film from about 115 to 135 nm at a measuring wavelength of 546 nm and a slow axis xcex4 which forms an angle (xcfx86RF3) from about 55 to 85 degrees with respect to the reference direction X in the counterclockwise direction when viewed from above; limiting an absorption axis xcex5 of the second polarizer which forms an angle (xcfx86pol2) from about 10 to 40 degrees with respect to the reference direction X in the counterclockwise direction when viewed from above; and limiting an angle formed by the slow axis xcex4 of the third retardation film and the absorption axis xcex5 of the second polarizer from about 30 to 50 degrees.
The method may also comprise limiting the birefringent retardation (xcex94ndLC) of the liquid crystal cell from about 700 to 730 nm at a measuring wavelength of 589 nm, limiting the angle (xcfx86pol1) formed by the absorption axis xcex1 of the first polarizer with respect to the reference direction X from about 40 to 50 degrees in the counterclockwise direction when viewed from above, limiting the birefringent retardation (xcex94ndRF1) of the first retardation film from about 155 to 185 nm at a measuring wavelength of 546 nm, and the angle (xcfx86RF1) formed by the slow axis xcex2 of the first retardation film with respect to the reference direction X from about 70 to 90 degrees in the counterclockwise direction when viewed from above, or limiting the birefringent retardation (xcex94ndRF2) of the second retardation film from about 360 to 400 nm at a measuring wavelength of 546 nm, and the angle (xcfx86RF2) formed by the slow axis xcex3 of the second retardation film with respect to the reference direction X from about 100 to 130 degrees in the counterclockwise direction when viewed from above.
The method may also comprise limiting an aperture ratio of each fine aperture from about 15% to 35% with respect to an area of one pixel pitch of the liquid crystal cell.
The method may also comprise driving the liquid crystal by a voltage averaging method and limiting the steepness index of the transmitted luminance vs. voltage characteristic of the liquid crystal composition of the liquid crystal layer from about 1.030 to 1.060 when driven by the voltage averaging method, driving the liquid crystal by a multi-line addressing method and limiting the steepness index of the transmitted luminance vs. voltage characteristic of the liquid crystal composition of the liquid crystal layer from about 1.040 to 1.075 when driven by the multi-line addressing method, or in general limiting a steepness index of a transmitted luminance vs. voltage characteristic of the liquid crystal composition of the liquid crystal layer to different ranges dependent on the method used to drive the liquid crystal layer.
The method may also comprise forming the high-reflectivity film such that the diffuse reflection surface has a high reflected luminance region with a substantially flat portion or forming the diffuse reflection surface to have curved surfaces with slopes that are discontinuous between adjacent curved surfaces and such that substantially no space exists between the adjacent curved surfaces. In the latter case, preferably the diffuse reflection surface is formed such that each curved surface has an asymmetrical sectional shape.